A low cost transceiver utilizing frequency modulation in a simplex or time-division duplex system may utilize a single precision oscillator locked to a high precision reference, for normal transmitter or receiver operation. Since transmitter and receiver functions of the transceiver are not simultaneously active in this system, a single oscillator is sufficient for normal operation.
System-on-Chip (SoC) transceivers implement on-chip tune, test and calibration (TTC) circuitry to minimize the cost of external factory equipment that would otherwise be necessary to perform these functions, and to enable these functions to be performed once transceivers are in use in the field. A method used to perform TTC on various stages in the transceiver is to inject a transmit path output into the receive path input, in a so called “loopback” topology, and then re-use receive digital circuitry to perform signal analysis for the TTC. A requirement for a periodic signal to be produced in the receive path and tested by receive digital circuitry during the TTC function, however, cannot be satisfied if the same oscillator is used to directly and simultaneously drive both the transmit and receive paths. An additional and different RF signal is therefore generated to drive the transmit path, exclusively for TTC functions. This additional RF signal may be supplied by an additional oscillator of the transceiver.
The requirement to create an additional high-quality RF signal is a design burden; the additional oscillator required to generate the RF signal adds silicon area (and, therefore, cost), and increases the peak power required to supply the transceiver, due to the power drawn by the additional oscillator and any additional circuitry associated with the oscillator, such as phase-locked loop circuits, during the TTC functions. That additional cost and peak power are associated with a circuit used only in TTC, and not while the transceiver is in normal operation, makes it very desirable to find an alternative to the two oscillator approach.